Physics Photo of the Week

October 28, 2011

Meteors or Satellites?
Photographing stars on October 16, 2011 to measure their brightnesses, one of the images had a bright streak at lower center.  On careful inspection there is a fainter streak near the center of the photo.  I initially thought I had "captured" two separate meteors within the 30 second frame.  Airplanes are ruled out because they usually leave a blinking trail.  I was able to conclude that these streaks both are caused by satellites because I had taken a series of 30 second exposures in immediate succession.  Meteors would only show up on one frame
because a meteor lasts only for about 1-2 seconds.  Satellites, on account of their orbits, cross the sky much more slowly passing the field of view in 2-3 30-second frames.

At right a sequence of highly-processed consecutive frames are shown.  The faint streak is visible in all three frames indicating the object took about 180 seconds to pass the field of view - definitely not a meteor.  The bright streak highly visible in the bottom of the middle image looks like a meteor that was visible for about 2 seconds during the 30 second exposure time for the middle image.  However, on close examination, the image immediately before that of the bright streak shows a "ghost" of a streak of the object that produced the bright streak as if the object moved from upper right towards bottom center.  This indicates that the bright object was also moving slowly like the faint object, but brightened during its pass through the middle frame.  This could be caused by reflected sunlight from a solar panel or a polished antenna as the angles between the Sun, spacecraft, and observer are just right. 

There are a number of communications satellites, called "Iridium" satellites that are well-documented and produce bright flares at predicted times.  From any location anyone can log onto certain websites and find predictions for flares produced by these satellites.  My new goal for a future PPOW is to photograph these documented flares.  I could not find a documented predicted flare for the evening of Oct. 16.  Some satellites are no longer maintained, and the predictions for flares have ceased.

On the photos, the bright object passes through the constellation Perseus just above the horizon.  The camera was tilted relative to the horizon. 

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to 

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